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Keys - Creating Chords from the Major Scale:

The theory behind figuring out chords that can be played in major keys is based off the major scale (likewise chords that can be played for minor keys are based off the minor scale). Basic Triads (chords with three notes) like major, minor, and diminished chords, are made up for the 1st, 3rd, and 5th scale degrees. The method for building these triads using the major scale is known as Stacking 3rds because each note is a 3rd apart.

The following examples will use the C major scale.
C D E F G A B
1 2 3 4 5 6 7 (scale degrees)

Starting with C, the 1 3 5 scale degrees would be C E G. This is a C major chord. These three notes are the only notes in a C major chord, but they can repeat as many times as you want.

To build more chords off of the C major scale, go to the next note in the scale, which is D. Starting on D, build the D chord using the 1 3 5 scale degrees counting D as the 1 scale degree and making sure to use the notes given in the C major scale. This D chord would be D F A, which is a D minor chord. Note: D major would have an F# for the 3rd scale degree and this chord has an F (try building a D major scale using the WWHWWWH step combination and you’ll see that the 3rd scale degree is an F#).

Repeat this process for all the notes in the C major scale....

C E G = C major
D F A = D minor
E G B = E minor
F A C = F major
G B D = G major
A C E = A minor
B D F = B diminished

Notice that all of the chords mentioned above only have notes from the C major scale. (This is very important!)

This is the idea behind keys. A key is essentially a group of cords that are built from the same scale... (For now we won’t worry about chords containing notes “outside” of the key.) In this example, the chords above were built using the notes from the C major scale so these are the chords that you could play in the key of C major.

Roman Numeral Numbers are used to indicate the chords in the key. Uppercase numbers are major chords and lower case numbers are minor chords.

Using C major as an example again:
C major = I
D minor = ii
E minor = iii
F major = IV
G major = V
A minor = vi
B diminished = viio

You will usually see an “o” after the Roman numeral to indicate that it is a diminished chord.

** This method can be applied to any key, but C major is the easiest to demonstrate, because it has no sharps or flats.